In pursuit of the best possible shift quality, many modern automatic transmissions use one-way sprag or roller clutches, often in series with a multi-plate clutch for establishing intermediate transmission gear ratios. Such constructions inherently provide high quality gear shifts with the least complicated control system. However, transmissions with such one-way clutches usually require yet another friction device for coast braking thereby making the transmission larger, heavier and costlier. For this reason, many modern automatic transmissions employ friction brake bands as grounding devices in place of one-way clutches even when desirable shift quality is sacrificed.
The friction brake band systems of such transmissions however are of compact construction and have less parasitic losses than multiple plate clutches when running in a released state. They also have an important functional advantage over the multi-plate clutches in that the torque capacity of a friction brake band in one direction (i.e. drive) is much higher than in the opposite direction (i.e. coast) making the calibration of power-on shifts off of a band much easier than off of a clutch, which has the same torque capacity in either direction.
In the event, the obtained shift quality from brake bands, is generally not as good nor as consistent as can be obtained with a one-way roller or sprag clutch. Poorly calibrated shifts with brake bands or multi-plate clutches, besides being uncomfortable to the driver and passengers, reduces the life of friction elements and can cause a premature failure of the transmission.
Normally, a friction brake band occupies far less transmission housing or gear box space than any functionally equivalent alternative. Often, some of this advantage is lost when a special radial bearing is required for supporting the brake drum to ground when side loads are applied to the drum by the band. The bearing is resultantly subjected to transversely directed brake actuating forces. The ideal position for this radial bearing for the drum is in the plane of these forces such that no cocking of the drum will take place when side loaded from apply of the band. This, however, precludes its placement over rotating gear units or other gear box components. Such distinct disadvantages severely limit the options available to a transmission designer pursuing the most compact, lightest and the least expensive gear box arrangement. Furthermore, misalignment caused by a side loaded brake drum can lead to gear noise problems and detract from the durability of the transmission.
A solution to such problems is found in the brake band system of this invention, in which all forces acting on the band are balanced and the brake drum is not subjected to any side loading when engaged. In this invention, the brake drum need not be supported directly to ground and only needs piloting like any other rotating transmission component. In one embodiment of this invention the brake band is split into two separate half-bands, each applied with an equal force acting in opposite direction so that an encompassed drum and attached gear element of a planetary gear set are held for reaction to condition the transmission for a predetermined drive ratio. In another preferred embodiment, the brake band comprises two individual full wrap bands: an inner full-wrap single strap band straddled by the side straps of an outer full-wrap band. In this case the apply and reaction lugs of the outer band connect the outer ends of the side straps to one another and form bridging over the single wrap inner band.
Both of these force balanced brake bands can be operated by a self-synchronizing actuating system based on the systems of my co-pending and parent patent applications, (1) PCT/US96/06415 filed May 7, 1996 which claims the priority of U.S. Ser. No. 08/436,756 filed May 8, 1995 for SELF-SYNCHRONIZING BRAKE BAND ACTUATING SYSTEM FOR AUTOMATIC CHANGE SPEED TRANSMISSIONS, and (2) U.S. Ser. No. 08/443,155 filed Jul. 12, 1995 for FORCE BALANCED BRAKE BAND WITH SELF-SYNCHRONIZING ACTUATING SYSTEM. Both of these applications describe a brake band actuating systems allowing freewheeler type power-on upshifts and downshifts for changing gear ratios without supplemental one-way roller or sprag clutches. Application Ser. No. 08/436,756 discloses a band unit, single or double wrap, for example, with one apply piston and one reaction piston and related controls. Application Ser. No. 08/443,155 discloses a band unit with half bands with separate reaction constructions and apply piston and controls.